How to snap to vertex on an unstraight/uneven axis?
You can keep using the knife tool then align the edge after, using "edge slide".

Edge slide is a way to grab along edges and is obtained by typing G twice.

So here:


make the cut
select the edge and use GG (double keys does not appears on the cast in the gif below) to align to the edge on the right.
then GG again to place it where you want, but this time the edge is parallel to the one on the right